This invention involves a system of components for hybridisation adapted to a technique for hybridisation by melting of solder projections, know as the xe2x80x9cflip-chipxe2x80x9d method, and which allows for uneven surfaces. The invention also involves a component substrate with hybridisation studs allowing for uneven surfaces.
In the context of the present invention, component means either an electronic component such as for example an electronic chip, an electronic circuit support, or an optoelectronic circuit support, or a mechanical component such as, for example, a cap or sensor of physical values.
The invention can be applied in the areas of electronics and optics in particular.
The invention can be used for example in the manufacturing of retinas for detection of electromagnetic waves, for hybridisation, on addressing circuits, laser matrices with vertical cavities for surface emission, or for hybridisation of optical detectors on reading circuits.
There are two main techniques for hybridisation of components by solder projections.
A first technique, called xe2x80x9ctechnique for hybridisation by meltingxe2x80x9d uses projections of meltable material such as an alloy of tin and lead SnPb or tin and indium SnIn for example. The technique of hybridisation by melting is illustrated by the appended drawings of FIGS. 1 and 2.
In these figures, the references 10 and 12 designate respectively a first and a second component to be interconnected. The first component 10 includes first studs 14a of a material wettable by the material of the projections of meltable material (solder). The studs 14a are surrounded respectively by an area 16a of material which is not wettable by the material of the projections. For the purpose of simplification, only two interconnection projections are shown in FIGS. 1 and 2. The components could however be equipped with a large number of projections.
In the same way, the second component 12 includes second studs 14b, also made of a material wettable by the projection material, and surrounded by an area 16b of non-wettable material.
The first and second studs 14a, 14b are associated to form pairs of studs. The studs of each pair of studs are essentially arranged face to face when the components to be hybridised 10, 12 are brought together with their hybridisation sides facing, as shown in FIG. 1.
The studs 14a of one of the components, in this case of the component 10, are equipped with projections 18 of meltable material. These projections 18 are to create a mechanical and/or electric link between the studs 14a, 14b of each pair of studs.
For a hybridisation, the component 12 is brought against the component 10 so that the studs 14b come into contact with the corresponding projections 18. The entire structure is then brought to the hybridisation temperature, greater than or equal to the melting point of the projections, in order to solder the projections 18 to the studs 14b. 
All of the projections 18 are thus soldered simultaneously to the studs of the component 12 which corresponds to them. After cooling, the structure in FIG. 2 is obtained.
The accuracy of the mutual positioning of the components during the placement of the component 10 onto the component 12 is not critical. During melting of the projection material, the components 10 and 12 are automatically aligned due to the surface tension of the molten projection material.
As indicated previously, due to the process of hybridisation by melting, all of the solder joints between the projections and the receiving studs are made simultaneously. The process of hybridisation by melting is thus particularly suitable for hybridisation of several components such as chips to a receiving component which forms the substrate. A high hybridisation yield can be obtained for these structures.
FIGS. 1 and 2 illustrate the technique of hybridisation by melting in the ideal case where the first and second components are perfectly flat. In the preceding description, the uneven surfaces which often appear on components were not taken into account. This surface unevenness generally results in deflection and has a greater influence on large components.
This situation is illustrated in FIG. 3.
A first component 110 has an essentially planar hybridisation side 111 equipped with projection receiving studs 120a, 122a, 124a, 126a and projections of meltable material 120b, 122b, 124b, 126b. A second component 112 has a hybridisation side 113 equipped with studs 120c, 122c, 124c, 126c for receiving the projections.
The second component is not perfectly planar but rather has a deflection curve f indicated in the figure.
The deflection is measured between a plane which goes through the base of stud 124c, i.e. the upper part of the component curve, and a plane parallel to the plane which goes through the base of stud 124c and passing through the furthest reception stud from the plan which is, in the case of the example of the figure, stud 126c. 
In FIG. 3, hb indicates the height of the projections which have not yet been soldered onto the receiving studs of the component 112. The height hb is measured from the hybridisation surface 111. h1 indicates the distance separating the first plane of the surface 111 after hybridisation of the components 110 and 112. The distance h1 is referred to in the remainder of the description as the xe2x80x9chybridisation height.xe2x80x9d
The absence of a connection between the studs 120c, 126c and their corresponding projections 120b, 126b is seen in FIG. 3. This flaw is due to the excessive mechanical deflection of the component 112.
Once the projections 122b, 124b are soldered to the receiving studs 122c and 124c, the hybridisation height h1 at the lowest point of the component 112, i.e. the point closest to the hybridisation surface 111 of the component 110, is defined.
One receiving stud of the component 112 does not come into contact with its corresponding solder projection if the distance F which separates it from the plane which goes through the base stud is such that h1+f greater than hb, i.e. f greater than hbxe2x88x92h1.
This is the case for studs 120c and 126c. 
The height of the solder projections can be easily calculated from the volume of meltable material from the projections and the wettable surface of the receiving studs 120a, 122a, 124a and 126a (sphere truncated by a disk). In the same way, the height h1 can be calculated by taking into account the volume of meltable material of the projections and the wettable surfaces of the receiving studs of each component (sphere truncated by two disks).
Considering that the volume of meltable material is essentially the same for all of the projections, it is possible to determine the maximum allowable deflection f before connection flaws of the projections appear.
Thus in effect F=hbxe2x88x92h1.
It is immediately apparent that this problem gets worse when the dimensions of the projections, and thus their height hb, is smaller. The problem is also aggravated for hybridisation of large components for which the deflection is naturally larger.
As an example, for hybridisation of a component with a matrix distribution of projections with a pitch of 25 xcexcm, the projections can have a thickness of 10 xcexcm between the receiving studs of each component (after hybridisation and a diameter of 15 xcexcm.
When, before hybridisation, these projections equip the receiving studs of a first component which has a diameter of 12 xcexcm, the height hb of the projections (in the form of spheres) not connected is hb=12.6 xcexcm. The hybridisation height h1 is, for the same data, such that: h1=10.4 xcexcm.
The maximum acceptable deflection f is thus 12.6xe2x88x9210.4=2.2 xcexcm. Components with a deflection less than such a maximum deflection are generally small. If the components are larger however, for example components with 2 cm sides, a deflection tolerance of 2.2 xcexcm is unacceptable. The price of flattening the components to respect such tolerances becomes prohibitive.
It is known that the maximum acceptable deflection can be increased by increasing the size of the hybridisation studs of one of the components to be hybridised.
More precisely, in a set of components to be hybridised, there is a first component for which the hybridisation studs equipped with projections of meltable material are a first area, and a second component with corresponding hybridisation studs which have a second area greater than the first one.
As the area of the wettable surface of the second studs is greater than that of the first studs, the hybridisation height as defined previously is reduced.
In addition, for a given volume of meltable material, the height of the projections before soldering remains high because these projections are formed on the first studs with a smaller area. The combination of these two effects allows for a greater deflection or greater unevenness of the components.
The advantage of increasing the size of the hybridisation studs of one of the components is illustrated by the appended FIGS. 4A, 4B and 4C.
FIG. 4A represents a partial section of a component 210, equipped with a first reception stud 214a with a surface area S1 and equipped with a projection 214b. The projection has a predetermined volume V and its height before hybridisation is noted hb.
FIG. 4B shows a partial section of a system of components 210 and 212 respectively equipped with receiving studs 214a and 214c, in disk form, and linked by a projection of meltable material 214b. FIG. 4B can be considered as illustrating the structure obtained by hybridising the component of FIG. 4A, equipped with the projection, with the second component 212.
The projection 214b which also has a volume V is soldered respectively to studs 214a and 214c which have a surface having essentially the same area S1. This corresponds to a typical situation. The hybridisation height measured on the hybridisation sides facing components 210 and 212 is indicated by h1 on FIG. 4B.
In the manner explained previously, the maximum admissible deflection in this case is f1=hb=h1.
FIG. 4C shows a partial section of a system of components 210, 212 of which the receiving studs have a wettable surface with a different area.
As in the preceding figures, the component 210 is linked to component 212 by a projection 214b of meltable material. The projection of meltable material is, as in FIGS. 4A and 4B, soldered to a receiving stud 214a, formed on the hybridisation surface 211 of component 210 and having area S1. The projection has a volume of meltable material equal to V.
However, component 212 has on its hybridisation side 213 a stud for receiving a projection, reference 214c, which has a surface with an area S2 greater than area S1.
FIG. 4C thus illustrates the structure obtained by hybridising the component in FIG. 4A, equipped with a projection of meltable material, with component 212, equipped with a projection receiving stud with a wettable surface of which area S2 is greater than area S1.
By comparing FIGS. 4B and 4C, it is seen that the hybridisation height h2 of the structure of FIG. 4C is less than the hybridisation height h1 of the structure in FIG. 4B.
For a given volume of meltable projection material, i.e. for a given height of projection hb before soldering, since the hybridisation height is lower (h2 less than h1), the maximum allowable deflection (f2=hbxe2x88x92h2) is thus greater (f2 greater than f1)
An illustration can be found in document (1) for which the reference is given at the end of the description.
The approach presented above suffers from a certain number of limitations linked to the increase in the number of hybridisation studs per unit of area for highly integrated components.
Designating as d the dimension in a given direction of the hybridisation studs for which the area is the greatest (d is for example the diameter) and as xcex94 the minimum spacing needed between the projections of meltable material for manufacturing reasons, the pitch p between the projections, in a given direction, must be such that
pxe2x89xa7d+xcex94
Estimating that xcex94 is greater than or equal to 3 xcexcm, it is impossible to compensate for a deflection of 4 xcexcm or more when the pitch between the projects (or the studs) is less than 15 xcexcm.
The appended FIG. 5 shows another possibility for making hybridisation studs.
A first component 310 is equipped with an xe2x80x9cordinaryxe2x80x9d hybridisation stud 314a, in the form of a disk, on which a projection 314b of meltable material is formed. The stud 314a has a surface with area S1.
The first component 310 is associated with a second component 312 equipped with a second hybridisation stud 315 with a surface of area S2 which is greater than S1. It is seen that the second hybridisation stud 315 has a recessed part in the form of a well 315a which allows for an increase in the size of the stud while reducing its crowding.
With a structure in accordance with FIG. 5, the pitch between the hybridisation studs can be reduced. The presence of a well 315a however reduces the hybridisation height h1 as defined previously (and indicated on FIG. 5). The available spacing between the components thus becomes very tight and good cleaning of the solder flux, which is necessary for remelting hybridisation techniques, becomes impossible. The little free space between the projections and the components prevents good cleaning of the flows.
At least a partial solution to the problems mentioned above can be found by using a second hybridisation technique referred to as the xe2x80x9cpressure hybridisation technique.xe2x80x9d
This technique starts with a structure comparable to that shown in FIG. 1. The component 12 is brought onto the component 10 by precisely aligning the studs 14b with the projections 18. Then, by applying appropriate forces to the components 10 and 12, the projections 18 are pressed firmly against the studs 14b to make a thermocompression bond.
This operation takes place at a temperature below the melting point of the projection material.
During pressure hybridisation, unevenness of component surfaces may be compensated for by controlling the hybridisation height. The hybridisation height is controlled in particular by crushing the projections to a greater or lesser extent. For a component with an uneven surface, the neighbouring projections at the summit of the curve caused by the unevenness are crushed until the most distant projections adhere to their corresponding receiving studs.
The pressure hybridisation technique has some drawbacks with respect to the melting hybridisation technique however.
For example, for pressure hybridisation the alignment of the components to be hybridised must be done with great accuracy. The auto-alignment phenomenon which exists for melting hybridisation does not occur at temperatures below the melting point of the projection material.
In addition, and unlike melting hybridisation, pressure hybridisation is unsuitable for simultaneous connection of many components during a given operation. The manufacturing yields are thus lower.
This invention proposes a system of components for hybridisation which avoids the problems mentioned above.
In particular, the invention proposes components for hybridisation allowing for hybridisation despite substantial unevenness compared with known structures.
Another purpose is to propose such components on which the pitch between the hybridisation studs can be reduced.
Still another purpose is to propose such components for which the hybridisation height remains sufficient to allow for good cleaning of solder flux.
In order to achieve these aims, the invention more precisely involves a system of components for hybridisation including a first component with a hybridisation side with first hybridisation studs, and at least a second component with a hybridisation side with second hybridisation studs, the first and second hybridisation studs being respectively associated in pairs of studs and arranged in places such that, on the first and second components, each second stud of a pair of studs is located essentially facing the first stud of the pair of studs when the first and second components are assembled with the hybridisation sides facing, one of the first and second studs, of at least one pair of studs, being equipped with a projection (also referred to herein as a convex bump) of meltable material and the other of the aforesaid first and second studs of the pair of studs, called the contact stud, having a surface made entirely of a material wettable by the meltable material. In accordance with the invention, at least one part of the contact stud forms a protuberance. In addition, the projections preferably have a quantity of meltable material sufficient to cover the protuberance during hybridisation.
The invention applies to all types of components, but in particular to components with a deflection or other surface unevenness.
The protuberance formed by the contact hybridisation stud allows for a substantial increase in the area of the wettable surface of the stud.
With the invention, the minimum hybridisation height is equal to the height of the protuberance formed by the contact stud(s) of the structures to be hybridised. By adjusting this height to a desired value, a sufficient spacing can always be had between the hybridised components to allow for good cleaning of the flux.
The maximum deflection which can be compensated for without producing a hybridisation flaw corresponds to the height hb of the projection of meltable material before hybridisation. It is sufficient that the extremity of the protuberance of the contact hybridisation stud, which faces the stud with the meltable material, reach the top of the projection of meltable material for an electrical and mechanical contact to be made.
Due to the invention it is no longer necessary to increase the diameter of the hybridisation studs to increase the area of their wettable surface. A greater density of hybridisation studs is thus possible.
The hybridisation studs with a wettable protruding surface can be in the form of a column which is essentially perpendicular to the hybridisation side of the component which bears these studs.
More generally, the protuberance of the stud can be a protrusion or elevation on the surface of the component of which the height, measured with respect to the hybridisation side, is greater than a maximum cross section. Maximum cross section means a maximum section measured essentially perpendicular to the protuberance. This is the case for a column of which the height is greater than the diameter which constitutes the cross section (maximum).
Based on one particular embodiment of a stud with a wettable surface which protrudes. This stud may have a base part in the form of a disk and a protruding part mounted on the base part.
According to a particular aspect of the invention, the height of the protuberance may be lower than the height of the projection of meltable material formed on the corresponding stud of the pair of studs.
The invention also involves a component substrate including at least one hybridisation stud of which at least one part protrudes.
Other characteristics and advantages of the invention will be clearer from the following description with reference to appended figures of drawings. This description is purely illustrative and in no way limiting.